Observational Epidemiological Studies

Question 1

How would you work out point prevalence?

Answer 1

(number of persons with disease at some time point) / (total population at risk of disease at the same time point)

Question 2

name 3 types of descriptive epidemiological studies

Answer 2

ecological studies
case reports / case series
cross-sectional studies

Question 3

What 2 broad things do we need to do when we assess if association is causal?

Answer 3

1 - consider bias, confounding, and chance as possible explanations

2 - consider the criteria for causality

Question 4

What 2 forms of bias might crop up in observed epidemiological studies?

Answer 4

selection bias

information bias

Question 5

What is selection bias?

Answer 5

occurs when the way in which groups of subjects are selected for a study means that the groups cannot be validly compared with each other (or with the general population)

Question 6

Name 4 examples of selection bias

Answer 6

sampling bias
response bias
healthy worker effect
healthy reproducer effect

Question 7

What is information bias?

Answer 7

when the way in which the data are obtained from groups being compared differs systematically

Question 8

Name 5 examples of information bias

Answer 8

recall bias
recording bias
interviewer bias
lost to follow up bias
social acceptability bias

Question 9

How would you work out incidence rate?

Answer 9

(number of new cases) / (number of person-years accumulated)

Question 10

How would you work out the risk?

Answer 10

(number of new cases) / (number of persons at risk at beginning of observation period)

Question 11

name 3 key functions of descriptive studies

Answer 11

To alert the medical community to what types of persons were most / least affected by disease

To assist in the evidence-based planning of health and medical care facilities

To provide suggestions concerning disease aetiology for further investigation using analytic studies (hypothesis generation)

Question 12

When would you use a cross-sectional study?

Answer 12

If in a particular geographical region we were to determine the point prevalence of a particular disease then a cross-sectional study is often undertaken
□ Determining need for specific health or social services
□ Development of hypotheses concerning risk factors for a disease

Question 13

Name 2 advantages of cross-secitonal studies

Answer 13

cheap

quick

Question 14

Name 3 disadvantages of cross-sectional studies

Answer 14

Time sequence and causation (disease and exposure measured simultaneously and so sometimes hard to interpret results in terms of cause and effect)

Cohort effects when interpreting relationships with age

Problems with interpretation of prevalence which is a mixutre of incidence and survival relating to disease

Question 15

What is an ecological study?

Answer 15

Studies in which the units of observation are groups of individuals rather than individuals themselves, for example, populations of different countries, or town, or health regions / districts

Question 16

What are the 2 main disadvantages of ecological studies?

Answer 16

The group of people described by disease data are often not the same people as those described by the exposure data

The time relationship between the measurements for exposure and disease is often unclear
(So we cannot assume from a relationship found amongst groups of individuals, that the same relationship also holds at the individual level)

Question 17

Name 3 advantages of Ecological studies

Answer 17

Quick
Inexpensive
Hypothesis generation

Question 18

What are cohort studies?

Answer 18

Cohort studies are studies in which a group of individuals who are defined on the basis of the exposure to a risk factor and are then followed up over time to determine who develops disease of interest

Question 19

What are the 2 types of cohort studies?

Answer 19

prospective

retrospective

Question 20

What are the 5 key advantages to cohort studies?

Answer 20

Provide estimates of absolute and cumulative risks of disease

Particularly useful for following up rare exposures

Eliminate some of the sources of bias which are unavoidable in case-control studies which rely on recall of past events i.e. if carefully conducted then recall bias is eliminated

Because exposure is measured before disease occurrence there is little doubt about cause and effect

Unlike case-control studies which address a single disease, there is no limit to the disease for which the risk of occurrence may be related to exposure

Question 21

What are the 3 key disadvantages to cohort studies?

Answer 21

Not useful for very rare diseases as few of the individuals exposed will develop the disease - but a case-control study design would be much more satisfactory

As they often involve following up large numbers of people over decades they are expensive and difficult to maintain, particularly from a funding stream perspective

If disease is rare and we only need relative risks associated with different levels of exposure to risk factors then unnecessarily expensive

Question 22

What might you use to choose the comparison cohort in a cohort study?

Answer 22

The general population and the use of SMRs and SIRs
National surveys
Another occupational group
Regional studies
A sub-cohort of the original exposed cohort

Question 23

How might you refine measurements of exposure?

Answer 23

Dichotomous (ever/never exposed)
Categorical (no/low/medium/high)
Quantitative measured exposure (continuous variable)

Question 24

What are the sources of measurement of exposure in a cohort study?

Answer 24

existing records
questionnaires to study subjects
clinical investigations

Question 25

What are the 2 main sources of bias in a cohort study?

Answer 25

healthy worker bias

lost to follow up bias

Question 26

What is the relative risk?

Answer 26

ratio between absolute risk in exposed and unexposed cohorts

Question 27

What is attributable risk?

Answer 27

different absolute risk in exposed and unexposed cohorts

Question 28

What is a case control study?

Answer 28

a group of individuals with a particular disease (the cases) are retrospectively compared with an appropriately selected group of controls in relation to their previous level of exposure to the risk factors of interest

Question 29

What is a nested study?

Answer 29

□ A study is nested (usually case-control studies) when it recruits study subjects from a population whose characteristics are known because its members are already the subjects of an existing larger study (for example, a cohort study or large RCT)

Question 30

What are the 3 main advantages to case control studies?

Answer 30

Effective use of resources to study the aetiological factors for rare diseases

Explore multiple exposures (risk factors) for a disease

Can estimate relative risk of disease associated with different levels of exposure to a risk factor through the odds-ration approximation to the relative risk

Question 31

What are the 3 main disadvantages to case control studies?

Answer 31

Recall bias e.g. abdo x ray in pregnant women and childhood cancer
People with cancer may remember things better

Address single disease only

Can’t absolute risks

Question 32

What are the steps of design with case control studies?

Answer 32

case definition
selection of controls
sources of controls
matching controls to cases
sources of bias

Question 33

where might you get controls for a case control study?

Answer 33

From the underlying cohort from which the cases were ascertained
Hospital controls
GP controls
Neighbourhood controls
Random digit dialing by telephone

Question 34

What are the main sources of bias in a case control study?

Answer 34

recall bias
recording bias
interviewer bias
response bias
sampling bias

Question 35

What problem is posed by case control studies in analysis and how is this navigated?

Answer 35

we don’t have incidence rate

there is an approximatino to the relative risk whihc can be estimated - the OR

Question 36

What does OR do?

Answer 36

is approximates RR which is better for rare disease

Question 37

How would you calculate OR?

Answer 37

(odds of disease in exposed group) / (odds of disease in unexposed group)

Question 38

What are the 9 Bradford criteria for assessing causality?

Answer 38

1 biological plausability
2 time
3 strength of association
4 biological gradient or dose-response relationship
5 consistency
6 specificity
7 coherence
8 experiments
9 analogy